Celery and Celeriac

• Seeds germinate in 10–14 days, and have full cotyledons by about 20 days.

Emergence is slow and the cotyledons are tiny. Later growth becomes more rapid, especially from about three weeks after transplanting.

Sowing conditions

• These seeds require light to germinate – this is important.

Simply scatter seeds on top of pre-moistened compost, with no compost on top of them. Seeds are small – be careful not to sow too many. Place a piece of glass over the tray, or put it in a bag of clear polythene, anywhere with warmth and some daylight.

The warmth requirement is around 20–25 °C/68–77 °F, which you could achieve on the windowsill of any room that has some heating in early spring.

• Germination does not happen if nights are much below 8 °C/46 °F, even when days are warm. Hence the benefit of house warmth, compared to a greenhouse where nights are much colder, even though sunny days are warm.

Sowing time

Mid-March in warmth is a reliable first sowing date. It’s the only one for celeriac, which needs the whole season to grow large. Buying plants in May is an option if you miss the March sowing, or don’t have a protected space for propagation. Celeriac sown after mid-April/mid-spring will grow nicely, but the harvest will be smaller.

Celery matures by early to midsummer, and can be sown again in May for later harvests.

• Two celery sowings give younger and more juicy stems at harvest because, although an early sowing will stand right through until autumn, its stems become more fibrous.
• New growth in late summer and through autumn, from the celery sowings of early spring, happens mostly as side shoots – see below.

In warm climates such as Zone 10, a third celery sowing is possible during summer, for harvests into winter, as long as frosts are only slight.

Sowing method

• Before sowing, water the compost to capacity.
• Scatter seeds on top and be careful not to sow too thickly.
• Use a cover of glass or polythene over the sown tray.
• Remove the cover once the first tiny leaves are all there.

Why sow in a tray and not in modules?

1. Seeds are tiny and almost impossible to sow individually or in twos. It’s much easier to scatter them on the surface of compost in a small tray.
2. Germination to the two-leaf stage takes 14–20 days, during which time one seed tray needs far less of your precious warm and protected space than would modules, and for the same number of seedlings.
3. After 20 days or so, it’s quick to prick seedlings into modules. The result is no empty module cells or uneven growth from patchy germination.

The cell size is best small, around 3 cm/1.2 in wide and 4 cm/1 in deep, because, as with carrots, growth at first is slow. A 60-cell module tray is ideal, and in spring you could, for example, grow 30 celery at one end and 30 celeriac at the other.

Seedlings benefit from the warmth of a greenhouse through April. They are not killed by frost if there is a cold night, but fleece over the trays will benefit them if that happens.

Pot on?

Potting on is not necessary, unless you are waiting for space to be available. You would do it at about six weeks from sowing, and into 7 cm/3 in pots.

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Transplant size

An excellent average size is 7 cm/3 in tall, while smaller is possible. Better than plants being too large and with a tightly grown root system, which results in a growth check after transplanting.

Transplant time and method

May, or late spring, is the month for transplanting both celeriac and your first sowing of celery. Neither are killed by frost, and you can transplant them earlier. However there is no rush: plants suffer some damage to leaves in frosts of around -2 °C/28 °F, while growth is stronger once soil is consistently warm, from late spring.

Subsequently, from late June to early July is good for setting out transplants from the second sowing of celery. They might follow, for example, early potatoes, spring onions, salads and peas. There is no ground preparation needed, apart from raking level and clearing any debris and weeds on the surface, assuming you had spread compost in winter or early spring.

• Use a wooden dibber to create holes a little deeper and wider than the module rootball. Push the transplant in firmly – the lowest part of stems may be below surface level. Don’t fill the hole because watering will do that eventually.

Spacing

• Celery can be as close as 22 cm/9 in each way, to have taller stems with fewer side shoots.
• Celeriac needs more space, up to 37 cm/15 in for large harvests, or 30–35 cm/2–14 in for medium-sized roots.

Interplanting

Between celeriac, you can interplant multisown spring onions, as in the photos below. They come to harvest within five to six weeks, just as the celeriac needs full space.

Growing care is mainly watering. After watering at planting time, give water again every two days if the weather is warm and dry. Then you may not need to water for a few weeks.

Celery

Start the main watering about a month after transplanting, and do it two or three times a week.

• Check soil moisture before watering – it may be drier than you expected if the celery is growing strongly.

Celery keeps sucking water out. It amazes me how much water the plants need, in order to grow quickly and with juicy stalks.

Water heavily in the ten days before harvest, unless rain is plentiful.

Celeriac

It needs much less water than celery, say just weekly in hot weather or on light soils.

• In an average temperate summer on heavy soil, you may hardly need to water celeriac.

Extra mulch to retain moisture?

For celery, in particular, slugs are a hazard. Compost mulch does not encourage them, so it’s best to use that only.

For celeriac, there is also a possibility of damage by woodlice, who breed fast in mulches with undecomposed wood and also in any decaying wood of old bed sides.

How to judge readiness

Harvest celery for tender stalks within four months of sowing, although they will stand for longer. The first pick can be of smaller heads after just three months. You can also twist, or cut off to eat, any larger side shoots from the outside of plants.

• Each sowing of celery can give six to eight weeks of harvest.
• Keep watering the remaining plants right up to harvest time.

Celeriac should be large enough to pull from October/mid-autumn and onwards. I recommend you harvest by Christmas, for less damage caused by disease, rodents and slugs.

How to pick

Use a strong and sharp knife to cut through the celery stem, just above soil level – each one is a lot of food. If not cleared, the root and stem base will grow baby celery which may or may not be worth it for the time and space needed.

Celeriac has many tenacious roots growing downwards from its hypocotyl. Use a sharp trowel, or even a spade, to slice through these, almost horizontally and just under the celeriac.

Small roots are fine to leave in the soil, while many others need trimming off the celeriac – see the photos for an idea of their number.

Cut off all leaves, without cutting into the root. After cutting to remove the main bulk of leaves, use the blunt back of a knife to rub off residual bases of leaf stalks.

Storing

Celery heads can be stood upright in a large cup or glass, with up to 2.5 cm/1 in of water. They draw on the water to keep their stems crisp. Cut off some celery to use as needed, for up to a week after harvest.

When storing celeriac, don’t trim the base too tight. Having some moist compost and soil between the many small roots helps to keep the tuber plump.

Celeriac survives some freezing, say to –5 °C/23 °F, but in autumn and early winter is more likely to suffer Septoria disease.

• Simply store in a box. You can eat celeriac any time until early spring – cut off all leaves and small roots.

Saving seed

Do this as for carrots: you need time, space, and skill

Which pests are likely, and when

• Slugs like celery, so, as with carrots, grow in clear open space, with minimal habitat for molluscs.
• Woodlice and slugs can tunnel into celeriac, so beds without wooden sides give best results.
• Rats and mice eat celeriac, which is a reason to harvest before winter and to store in a rodent-free place.

Protections needed

Keep any nearby slug habitats to a minimum.

Use only compost as mulch, practice no dig, and have no decaying wooden sides to beds.

Other difficulties

A big problem can occur in autumn’s damp weather: ‘late blight’, or Septoria apiicola. It starts with a few brown spots on leaves, followed by rotting of large leaf areas.

• Eventually plants stop growing, while the Septoria stains celery stalks and causes celeriac roots to turn soft and mushy.  
• I find it’s as hard to control as late blight on tomatoes, and removal of infected leaves makes little difference.
• In damp weather, from late summer, water less and only at soil level, and avoid watering in the evening. You want leaves to have a chance to dry from any splashing after you have mainly watered the roots.